Electrolytic cell bath composition for production of magnesium

ABSTRACT

AN ELECTROYLTIC CELL BATH COMPOSITION FOR PRODUCTION OF MOLTEN MAGNESIUM CONTAINS ABOUT 6.36.5 PERCENT, BY WEIGHT, OF MAGNESIUM CHLORIDE WITH ESSENTIALLY THE REMAINDER BEING LITHIUM CHLORIDE AND BARIUM CHLORIDE IN A WEIGHT RATIO TO EACH OTHER SUFFICIENT TO PROVIDE THE COMPOSITION WITH IMPROVED ELECTRICAL CONDUCTIVITY AND A SPECIFIC GRAVITY HIGH ENOUGH TO CAUSE MOLTEN MAGNESIUM FORMED THEREIN DURING ELECTROLYSIS TO FLOAT THEREON. A WEIGHT RATIO OF BARIUM CHLORIDE TO LITHIUM CHLORIDE OF AT LEAST ABOUT 1:7.5 IS EMPLOYED.

United States Patent 3,630,859 ELECTROLYTIC CELL BATH COMPOSITION FORPRODUCTION OF MAGNESIUM James G. Macey, Salt Lake City, Utah, assignorto Pete Prestininzi, Long Beach, Calif., and Ruth G. Macey and Anne M.Macey, with rights of survivorship, fractional part interest to each NoDrawing. Filed Feb. 16, 1970, Ser. No. 11,832

Int. Cl. C22d 3/08 US. Cl. 204-70 7 Claims ABSTRACT OF THE DISCLOSURE Anelectrolytic cell bath composition for production of molten magnesiumcontains about 636.5 percent, by weight, of magnesium chloride withessentially the remainder being lithium chloride and barium chloride ina weight ratio to each other sufficient to provide the composition withimproved electrical conductivity and a specific gravity high enough tocause molten magnesium formed therein during electrolysis to floatthereon. A weight ratio of barium chloride to lithium chloride of atleast about 1:75 is employed.

BACKGROUND (1) Field of the invention This invention relates toelectrolytic cell bath compositions, particularly those utilized for theproduction of molten magnesium from magnesium chloride.

,(2) Prior art Magnesium metal is produced most economically by theelectrolysis of molten magnesium chloride that exists in an electrolyticcell as a portion of a molten mixed salt bath, the other salts in thismolten bath mix or melt being, conventionally, chlorides of sodium,potassium, and/or calcium. Electrical conductivity of such a melt is solow as to interpose severe restrictions on the possible distance betweenthe anode and the cathode of the cell. If a wider spacing could be usedbetween the anode and cathode, a stronger and more rugged cell designhaving a much longer operating life could be employed.

It has recently been discovered that a high content of lithium chloridein the magnesium electrolytic cell bath, preferably upwards of 60percent by weight, more than doubles cell conductivity, reducing powerrequirements, per unit of magnesium metal produced, by more than onethird. Anode to cathode distance can be much greater, permitting a farstronger cell design with a much longer operating life before a completerebuilding is necessary. However, lithium chloride in highconcentrations in such cells lowers the specific gravity of the cellbath to such an extent that the molten magnesium produced sinks to thebottom, mixes with oxide sludge conventionally present and requiresspecial means for recovery in purified form. It would be highlydesirable to lower capital plant costs, production costs and plantmaintenance costs by providing an improved way of obtaining magnesiumelectrolytically from magnesium chloride.

SUMMARY OF THE INVENTION Such an improved way has now been discovered.It employs the improved electrolytic cell bath composition described inthe abstract above. The composition is readily usable in a cell ofconventional design which keeps chlorine rising out of the cell melt atthe anode from mixing with magnesium metal forming at the cathode. Sucha feature is, conventionally, a refractory semi-wall of, for example,silica brick or the like, extending from the roof of the cell down intothe melt, preferably by several inches.

Patented Dec. 28, 1971 A substantially thicker semi-wall than the onesnow conventionally used can be employed, because of the greater cellbath composition conductivity (more than double the conventional valuefor cells containing the usual mix of chlorides of sodium, potassiumand/ or calcium along with magnesium). Accordingly, such a cell has alonger operating life before it becomes necessary to shut down the celland rebuild the wall. Both maintenance costs and cell downtime arethereby minimized.

Due to the doubled electrical conductivity of the cell, electric powerrequirements, per pound of magnesium metal produced, are reduced by morethan one third.

Of considerable importance, a cell melt is employed which is heavyenough to insure that the molten magnesium metal being formed by theelectrolytic action, floats upon the cell melt. This avoidsintermingling the pure magnesium metal with high magnesium oxide sludgewhich may form on the cell bottom if moisture is present in themagnesium chloride fed to the cell, as is usually the case. Secondaryseparation of the metal from the sludge is thus avoided, savingequipment and processing costs.

The highly conductive cell bath composition of the invention facilitatessubstantially higher production of magnesium metal and chlorine from acell of given dimen sions. Thus, more metal is produced from a totalmagnesium reduction plant of any given size, and capital requirementsfor a given capacity of magnesium metal per year will be substantiallyreduced.

EXPLANATION AND EXAMPLES The achievement of these substantial operatingand capital investment economies requires that lithium chloride be usedto optimum concentrations in a magnesium reduction cell bath wherein themolten magnesium metal being formed will float. Of course, magnesiumchloride, preferably in anhydrous form, should be present in asufiicient concentration in the composition so that economicalproduction of magnesium occurs. It has been determined that magnesiumchloride should be present in a concentration of at least about 6percent by weight of the composition in order for the process to beeconomical. Loading of the composition with magnesium chloride up toabout 36.5 percent by weight of the composition can be tolerated.Accordingly, the useful range for magnesium chloride concentration inthe composition is about 6-365 percent, by weight of the composition. Inorder to prevent the thus formed pure magnesium metal from sinking inthe cell hath made lighter in specific gravity by high concentrations oflithium chloride, a cell bath composition is employed which makesadequate but minimal use of the heavy inorganic chloride, bariumchloride, to raise the specific gravity of the cell melt to a level wellabove that of the molten magnesium while permitting use of lithiumchloride in percentages by weight that range upwards from 56 percent toassure the highest possible electrical conductivity. A specific gravitysufiiciently higher (at least about 0.04) than that of molten magnesiummetal (1.557) at a suitable operating temperature, for example,approximately 800 C. is achieved by using lithium chlo ride in aconcentration not in excess of about seven and a half (7.5) times thatof barium chloride in the composition. Such a ratio assures the requiredspread between the specific gravity of the melt and the molten magnesiumfor all conventional cell operating temperatures, for example 700 C.-950C. A practical operating range is usually about 750-850 0, preferablyabout 800 C.

It will be understood that any conventional type of barium chloride,magnesium chloride and lithium chloride can be used in the composition,preferably pure and anhydrous.

The following specific example further illustrates certain features ofthe invention.

3 EXAMPLE An electrolytic cell is employed to convert magnesium chlorideto magnesium for recovery thereof in purified form. The cell is ofconventional design such as is currently employed in electrolyticmagnesium production in Norway. It employs steel cathodes and 4-inchthick graphite anodes separated by acid brick divider walls disposedwithin a hollow brick chamber having a ceramic top and chlorine exitconduits adjacent the upper end of each anode. A cell melt compositionis employed which, however, drastically differs from that conventionallyemployed in such cell. Instead of a conventional mix of magnesiumchloride and sodium chloride and/or potassium chloride and/or calciumchloride, the cell employs the composition of this invention, the ratioof constituents of which range between the extremes shown in the tableset forth below, decreasing during operation of the cell from thehighest value shown for the magnesium chloride (composition V) to itslowest value (composition I) before additional magnesium chloride isused to replenish the dwindling supply in the melt.

The cell is operated at about 800 C. with a power requirement less thanthat that of conventional cells employing conventional cell melts.Anhydrous magnesium chloride is periodically added to the melt in theconven tional manner and pure molten magnesium is periodically passedfrom the top of the melt and out of the cell. Chlorine gas is keptseparated from the molten magnesium and passes through the exit conduitsfrom the cell. This cell operates continuously for more than a yearwithout repair and in an economical manner.

In a second run, employing the same cell and bath composition, exceptfor an operating range of about 750-850" C. and the substitution ofhydrous magnesium chloride for anhydrous magnesium chloride, somemagnesium oxide sludge forms on the bottom of the cell and isperiodically withdrawn, but does not mix with the pure molten magnesiumat the top of the melt, so that contamination of the thus formedmagnesium does not occur.

When bath compositions outside the range demonstrated by the table aboveare employed, the advantages of the invention are not obtained,specifically reduced power consumption and flotation of the moltenmagnesium. Lithium chloride concentrations below about 56 percent resultin sharp increases in power consumption. The cell is operated mostefficiently with lithium chloride levels above about 60 percent. Whenthe magnesium chloride content drops below about 6 percent, the celloperates inefficiently and when the magnesium chloride content exceedsabout 36.5 percent, suflicient barium chloride must be added to the meltto keep the molten magnesium from sinking in the melt so that thelithium chloride content again falls below that level which affordsimproved electrical conductivity and resulting reduced powerconsumption. When the lithium chloride concentration in the melt exceedsabout 83 percent while retaining at least about 6 percent magnesiumchloride, the barium chloride content is insufficient to increase thespecific gravity enough to assure proper flotation of molten magnesium.Accordingly, the barium chloride weight ratio to lithium chloride shouldbe maintained at at least about 1:75 with a barium chloride range ofabout 7.5-11 weight percent, and a lithium chloride range of about 5683weight percent, the magnesium chloride proportionately ranging inconcentration between about 6 and 36.5 weight percent, as previouslydescribed and as set forth in the table above.

Various changes, modifications and alterations can be made in thepresent composition. All such changes, modifications and alterations asare within the scope of the appended claims form part of the presentinvention.

What is claimed is:

1. An electrolytic cell bath composition for production of moltenmagnesium which composition contains about 636.5 percent, by weight ofsaid composition, of magnesium chloride, the balance of said compositionconsisting essentially of lithium chloride and barium chloride in aweight ratio to each other sufiicient to provide said composition withimproved electrical conductivity and a specific gravity sufiicientlyhigh to cause molten magnesium formed therein during electrolysis tofloat thereon.

2. The composition of claim 1 wherein said barium chloride and lithiumchloride are present in a weight ratio to each other of at least about1:7.5.

3. The composition of claim 2 wherein said lithium chloride is presentin a weight concentration in excess of about 56 percent and wherein saidmolten magnesium chloride floats on said surface at bath temperatures inexcess of about 800 C.

4. The composition of claim 2 wherein said lithium chloride is presentin a weight concentration in excess of about 60 percent and wherein thespecific gravity of said composition at 800 C. is at least about 1.62.

5. The composition of claim 2 wherein said lithium chloride is presentin a concentration of about 83 percent, by weight of said composition,wherein said barium chloride is present in a concentration of about 11percent, by Weight of said composition, and wherein said magnesiumchloride is present in a concentration of about 6 percent, by weight ofsaid composition, and wherein the specific gravity at 800 C. of saidcomposition exceeds that of molten magnesium at that temperature byabout 004+.

6. The composition of claim 1 wherein only magnesium chloride, lithiumchloride and barium chloride are present, except for trace impurities.

7. The composition of claim 2 wherein only magnesium chloride, bariumchloride and lithium chloride are present, except for trace impuritiesand wherein the specific gravity of said composition at 800 C. is atleast about 1.6.

References Cited UNITED STATES PATENTS 3,317,414 5/1967 Foulgner 204-3,389,062 6/1968 Love 20470 3,418,223 12/1968 Love 20470 JOHN H. MACK,Primary Examiner D. R. VALENTINE, Assistant Examiner

